The present invention relates to a process for preparing sulphuric methacrylamide from acetone cyanohydrin and also to a device for carrying out this process.
Sulphuric methacrylamide or methacrylamide sulphate [CH2xe2x95x90C(CH3)xe2x80x94CONH2,H2SO4] is an intermediate product which gives methyl methacrylate by reaction with methanol and water.
It is known practice to prepare sulphuric methacrylamide from acetone cyanohydrin and concentrated sulphuric acid.
This preparation generally comprises the reaction of acetone cyanohydrin with concentrated sulphuric acid to give xcex1-hydroxyisobutyramide and xcex1-hydroxyisobutyramide sulphate. These two compounds are then heated to give sulphuric methacrylamide.
Such a preparation is the subject of French patent No. 1 529 440 in which is described a process with staging of the introduction of acetone cyanohydrin, comprising a step in which acetone cyanohydrin is mixed with sulphuric acid using a sulphuric acid/acetone cyanohydrin molar ratio at least equal to 1.5, a step in which the mixture obtained above is subjected to a dehydration by heating, acetone cyanohydrin is then added to the reaction medium so as to again have a sulphuric acid/cyanohydrin molar ratio (of greater than 1) and, finally, the reaction mixture is again subjected to a dehydration by heating.
European patent application No. 226 724 relates to a process for preparing methacrylamide from acetone cyanohydrin and concentrated sulphuric acid, in which a two-phase liquid mixture consisting of an inert, linear, saturated hydrocarbon containing from 5 to 7 carbon atoms and acetone cyanohydrin are introduced into sulphuric acid.
European patent application No. 999 200 relates to apparatus and a process for the high-yielding production of methyl methacrylate or of methacrylic acid, in which a piston-type cracking reactor is used for the thermal conversion.
In the translation of the article published in the Russian journal Zhurnal Prikladnoi Khimii, Vol. 47, No. 6, pp. 1347-1351, June 1974, entitled xe2x80x9cVarious reactions occurring in the synthesis of methyl methacrylate from acetone cyanohydrin and oleumxe2x80x9d by A. A. Michurin, E. A. Sivenkov, E. N. Zil""berman and T. I. Tret""yakova, it is demonstrated that the reaction of acetone cyanohydrin with sulphuric acid to give xcex1-hydroxyisobutyramide sulphate provides better results when it is carried out with a 1/2 cyanohydride/oleum molar ratio in the presence of oleum containing 5 to 10% sulphur trioxide. However, to limit the appearance of side reactions during the subsequent stoving step, it is recommended to work with an oleum containing 0-3% sulphur trioxide.
The aim of the invention is to propose a process for preparing sulphuric methacrylamide which has a high yield even when a small excess of sulphuric acid is used, i.e. when the overall sulphuric acid/acetone cyanohydrin molar ratio is low.
The process according to the invention comprises the following steps:
a) a mixing step, in which acetone cyanohydrin is mixed with sulphuric acid which is free of sulphur trioxide;
b) a stoving step, in which the mixture obtained above is subjected to a dehydration by heating;
c) a step of adding further acetone cyanohydrin, in which more acetone cyanohydrin is added to the reaction medium, without adding more sulphuric acid, and with mixing;
d) another stoving step, in which the mixture is subjected to a further dehydration by heating;
e) optionally, one or more additional step(s) of further addition of acetone cyanohydrin followed by an additional stoving step;
f) optionally, a final stoving step; and it is characterized in that sulphur trioxide is also introduced in step c) and/or just after this step c) and/or, where appropriate, in one or more of the additional steps of further addition e) and/or just after this or these step(s) e).
Such a process thus has the advantage of reducing the number and extent of the side reactions. In addition, it allows the use of starting reagents that are not necessarily anhydrous.
A subject of the invention is also a device for carrying out the process according to the invention.